I have two 6950s with XSPC waterblocks. I was having issues getting crossfire to run so I decided to pull each card and test them one at a time with the waterblocks installed. I came to the conclusion that one card was not working correctly since it just gives me a black screen in furmark and the system locks up.

So to confirm that the card was in fact dead I re-installed the stock cooler and low and behold the card worked perfectly fine. So I figured that it was just a bad mount of the waterblock causing the issue so I reinstalled the block and I am still having the same issue. Anybody have any tips for me since I am going nuts trying to figure out why this is happening.

Now just to confirm, I have tested both cards in both PCI-E slots both with and without waterblocks on multiple different driver sets. One card is perfectly fine both with the unlocked shaders and locked shader bios with either the water block or reference cooler in both slots.

The other card black screens in fur mark, locks up in catalyst control center, and black screens in every game in both slots with the water block on using either the default bios or unlocked shader bios. But when I slap the reference cooler back on it works fine in both slots with both bios.

Have you tried mounting the block with less tightening? Does anything mounted on the PCB seem to be touching the block anywhere? I've heard of this happening before and often it's due to the block warping the PCB in a way that something shorts out.

I have loosening all the screws a bit, I have also tried moving the thermal pads around, in total I have re-mounted the block about 6 times and each time I am having the exact same issue. It is almost like the card won't switch to 3D clocks and it causes a black screen. I have even tried connecting the stock cooling fan to the header to eliminate that as a potential cause.

I have looked very closely at the block and there is nothing I can see that is touching anywhere, it is somewhat hard to tell with the block mounted but there is no evidence that there is a short causing the problem.

When I put the stock cooler on it works perfectly fine, I guess my last option would be to take the block off the working card and try it on this one. It is just very frustrating and time consuming to keep trying different solutions.

Oh and the board shows zero signs of bowing or warping and all the standoffs are making proper contact with the card.

the stock retention bracket uses much smaller screws and the backplate gets in the way of the fittings. So I cannot try that, too bad because that would be a good way of eliminating potential mount problems.

It is a reference Sapphire 6950, and I have tried it with the fan plugged in and that didn't make a difference.

I have also tired both blocks on the card and the same result with each.

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well Id say its sapphire messing about with warranty. their warranty policy states that taking off their own cooler voids warranty. So if something is genuinely wrong with the cards. you cant send them back.

theres been a lot of speculation about how partners like sapphire, XFX, HiS etc etc tell that you have removed the stock cooler. some of them hide stickers under the cooler where you cant get to it that 'breaks' if you remove the cooler, and some just stick small void warranty on stickers over screws to stop you tinkering.

It wouldnt suprise me if they rigged the card up top stop working properly if you removed the stock cooler from it and thats what seems to be the case here.

its too much of a coincidence that it happend to BOTH cards from the same partner.

Actually this is a possibility, It seems to do it whenever the card tries to switch to 3D clocks. The strange thing is, the system doesn't crash, just whatever program I try to run. If i kill the process through task manager then the system usually recovers the display driver.